The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube equipped with an in-line type electron gun which is its focusing characteristics drastically improved by enlarging the equivalent aperture.
The color cathode ray tube, which is much used as a display device in TV receivers and terminals of information devices, still requires drastic improvement in its focusing characteristics in order to provide higher precision and improved quality of display images.
The factors which exert serious influences upon the focusing characteristics of color cathode ray tube are exemplified by the magnifications and aberrations of the main lens of the electron gun of the color cathode ray tube.
In a color cathode ray tube, the distance from the main lens to the focal plane (or fluorescent face) is determined when the scanning area and the maximum deflection angle of the electron beam are determined. The lens magnification is reduced if the lens converging action is as, for example, by enlarging the diameter of the aperture of the electrodes constituting the main lens as much as possible, under the condition that the distance to the focal plane is constant. Further the angle of the incidence of electron beam upon the main lens is reduced if the divergence of the electron beam in the main lens is suppressed within a predetermined value so as to prevent an increase in deflection errors.
If the electron beam incidence angle is designated at .alpha.i, the minimum disturbance circle diameter .delta. of the electron beam by the most dominant spherical one of the aberrations of the main lens is expressed by the following equation: EQU .delta.=(1/2)M.multidot.Csp.multidot..alpha.i.sup.3,
wherein:
M: lens magnification; and PA1 Csp: coefficient of spherical aberration.
Thus in the electron gun of the cathode ray tube, the lens magnification and the spherical aberration are reduced to improve the focusing characteristics if the converging action of the main lens is weakened.
One method of weakening the converging action of the main lens is to enlarge the diameter of the aperture of the electrodes constituting the main lens as much as possible.
However, the enlargement of the diameter of the aperture of the main lens constituting electrodes thickens the neck portion accommodating the electron gun so that the deflection yoke to be used is necessarily enlarged, causing an increase in the deflecting electric power.
FIG. 18 is a schematic section for explaining the construction of an electron gun used in the color cathode ray tube of the prior art, which has been proposed to enlarge the diameter of the aperture of the main lens constituting electrodes with respect to the diameter of the restricted neck portion. Reference numeral 10 designates cathodes; numeral 11 a first grid electrode (i.e., G1 electrode); numeral 12 a second grid electrode (i.e., G2 electrode); numeral 13 a third grid electrode (i.e., G3 electrode); numeral 14 a fourth grid electrode (i.e., G4 electrode); numeral 15 a fifth grid electrode (i.e., G5 electrode); numeral 16 a sixth grid electrode (i.e., G6 electrode); numeral 17 a shield cup; numeral 15' an internal electrode of the fifth grid electrode; numeral 16' an internal electrode of the sixth grid electrode 16; reference D5 an amount of regression or recessing of the internal electrode 15' with respect to the face of the G5 electrode 15 opposing the G6 electrode 16; and reference D6 an amount of regression or recessing of the internal electrode 16' with respect to the face of the G6 electrode 16 opposing the G5 electrode 15.
In the in-line type electron gun having three electron beams BR, BG and BB arrayed horizontally with a gap S, as shown in FIG. 18, the electrodes constituting the main lens are arranged such that they confront the two cylindrical electrodes (i.e., the fifth grid electrode 15 and the sixth grid electrode 16) having a flattened single aperture with its longer axis in the (in-line) direction in which the three electron beams BR, BG and BB are arrayed.
FIGS. 19(a) and 19(b) are front elevations taken in the fifth grid electrode direction along the M--M line of FIG. 18. FIG. 19(a) is an explanatory view of the main lens aperture in the case of a large S dimension (i.e., the distance between the electron beam orbits taken in one direction or the in-line array direction, that is, the distance between the center electron beam BG and the side electron beams BR and BB), and FIG. 19(b) is an explanatory view in the case of a small S dimension as compared with the case of FIG. 19(a).
Incidentally, in a front elevation of the sixth grid electrode, as taken along line N--N of FIG. 18, the reference numeral 15 in FIGS. 19(a) and 19(b) is replaced by numeral 16.
Here, in the example of FIG. 18, as shown in FIGS. 19(a) and 19(b), the flattened shape of the aperture of the aforementioned fifth grid electrode 15 and sixth grid electrode 16 (although not shown in FIGS. 19(a) and 19(b)) is not circular but is formed by joining two semicircular arcs by two parallel straight lines. However, the aperture should not be limited thereto if it is flattened to have its longer axis in the in-line direction.
Since such non-circular main lens has a larger diameter in the horizontal direction than in the vertical direction, the invasion of the electric field is more in the horizontal direction so that the effective diameter is larger in the horizontal direction than in the vertical direction. As a result, the lens converging action is strengthened in the vertical direction so that an astigmatism will appear when the electron beams are to be converged. Incidentally, this prior art is disclosed in Japanese Patent Publication No. 18540/1990.
As shown in FIGS. 19(a) and 19(b), therefore, the astigmatism is corrected by the internal electrodes 15' and 16' which are disposed in the cylindrical electrodes (i.e., the fifth grid electrode and the sixth grid electrode) 15 and 16 for allowing the three electron beams to pass therethrough and which are formed with elliptical apertures 15.sub.2 and 16.sub.2 (although the latter 16.sub.2 is not shown) having their longer axes in the vertical direction (perpendicular to the aforementioned one direction).
An effectively large aperture lens is formed while suppressing the aforementioned spherical aberration and astigmatism, by adjusting the shape and dimension of the elliptical apertures and the mounting positions (i.e., the amounts of regression or recessing from the confronting faces of the two electrodes) of those internal electrodes 15' and 16', as shown in FIG. 18.
Moreover, spherical aberration and astigmatism can be suppressed by adjusting the positions of the internal electrodes which are mounted in the two electrodes constituting the main lens, and the three electron beams BR, BG and BB can be directed to converge on the fluorescent face by deflecting the side electron beams BR and BB toward the center electron beam BG.
A color cathode ray tube having an electron gun of this kind is disclosed in the aforementioned Publication and Japanese Patent Publication No. 44379/1992, for example.
With the construction described above, the shorter gap (i.e., the S dimension) of the three electron beams is the more convenient for achieving a larger aperture lens in the in-line electron gun.
Here will be examined the correspondence between the S-dimension in the main lens portion of the electron gun and the aperture shapes of the cylindrical electrodes 15 and 16 with reference to FIGS. 19(a) and 19(b) (as taken in section M--M of FIG. 18). The horizontal aperture dimension H can be expressed, as follows: EQU H=2(R+S).
Here, if the aperture dimension V in the vertical direction is substantially equalized to 2R and if the positions and shapes of the internal electrodes 15' and 16' are adjusted, the effective lens apertures for the center and two side electron beams can be equalized substantially to 2R in the vertical and horizontal directions.
If an in-line type electron gun having the aforementioned construction and which is used in a color cathode ray tube having a nominal frame size of 14 to 25 inches, for example, and a neck external diameter of 29 mm is to be accommodated in the cathode ray tube having the neck external diameter of 29 mm, the aforementioned H dimension is limited to about 19 mm including the thickness of the electrodes and the gap from the neck internal wall.
With an equal neck diameter, that is, with an equal horizontal aperture dimension H, as apparent from the comparison between FIG. 19(a) and 19(b), the aperture diameter "2R" of the main lens for the center and two side electron beams becomes more for the smaller S dimension, as shown in FIG. 19(b), than for the larger S dimension, as shown in FIG. 19(a). As a result, the construction of FIG. 19(a) has a greater spherical aberration and astigmatism of the main lens than the construction of FIG. 19(b) so that its focusing characteristics are worse.
This means that the S dimension is desirably set to a smaller value so as to provide an electron gun having excellent focusing characteristics. Despite this desire, however, with the smaller S dimension, the two side ones of the three electron beams are reduced in their incidence angle upon the shadow mask, as described above. This further means that the distance (which will be called "Q") between the shadow mask and the fluorescent face has to be enlarged.
The space between the electron guns and the shadow mask is shielded from the influence of the earth magnetism by a shadow mask and the magnetic shield. With a large Q dimension, however, the section in which the electron beams are influenced by the earth magnetism is elongated. As a result, even if the color cathode ray tube is directed in one direction and adjusted to cause the electron beams to land on the correct position, the electron beams are moved by the influence of earth magnetism, when the color cathode ray tube is directed in another direction, so that the electron beams fail to land on the correct position, thereby to deteriorate the color purity of the color cathode ray tube.
In the invention disclosed in Japanese Patent Laid-Open No. 123288/1983 or 232387/1991, the means for correcting the aforementioned influence of the earth magnetism is exemplified by a correction coil disposed around the panel portion of the color cathode ray tube for bucking the external magnetism (i.e., the horizontal component of the earth magnetism) in the axial direction, thereby to suppress the purity deterioration.
In Japanese Patent Laid-Open No. 104187/1980 or 78388/1990, on the other hand, there is disclosed a color cathode ray tube which is equipped with a correction coil for bucking the vertical component of the earth magnetism.
In the case of the prior art cathode ray tube having a neck external diameter of 29 mm, an electron gun of the type having a cylindrical lens of a diameter of about 5.5 mm, for allowing the three electron beams to pass therethrough in the main lens portion, has an S dimension of 6.6 mm. This S dimension is narrowed to 5.5 mm in the electron gun of the aforementioned type disclosed in Japanese Patent Publication No. 18540/1990 or 44379/1992.
FIG. 20 is an explanatory diagram of a relation between the S dimension and the purity and plots, which diagram electron beam landing degree (.mu.m) against the S dimension (mm).
FIG. 20 plots the relation between the electron beam landing degree and the S dimension, which was experimentally obtained at the central portion of the display when a highly fine color cathode ray tube (the shadow mask of which had a pitch of 0.28 mm) having an effective display diagonal dimension of 36 cm and a deflection angle of 90 degrees for an information processing terminal was turned in the east-west direction to the north-south direction.
Incidentally, the electron beam landing degree indicates the distance from the end portion of the fluorescent element of another color to the end portion of the electron beam when the electron beam center is shifted from the center of the fluorescent element for the electron beam to land by the aforementioned turn so that it approaches the adjoining fluorescent element of another color.
Since this electron beam landing degree is smaller in the peripheral portion than at the central portion of the display, the purity is liable to deteriorate if the electron beam landing degree becomes lower than 7 .mu.m.
It is found from FIG. 20 that the S dimension of about 4.8 mm is required for retaining the electron beam landing degree at 7 .mu.m or higher, while considering the production deviation, so as to prevent the deterioration of the purity in the aforementioned peripheral portion of the display.
As a result, if the value of the aforementioned dimension H is at about 19 mm, the distance R from the center of the side electron beam to the inner wall of the electrode will be about 4.7 mm, and the enlargement of the distance R will be limited to about R.apprxeq.S.
The value (i.e., the R dimension) of the distance R indicates the shortest distance from the center of the side electron beam to the inner wall of the electrode and accordingly gives the effective radius of the main lens of the electron gun in the outward direction with respect to the side electron beam.
In the main lens of the aforementioned electron gun disclosed in Japanese Patent Publication No. 18540/1990, the elliptical aperture shapes and mounted positions (i.e., the positions of regression or recessing from the two confronting electrodes, as indicated by the dimensions D5 and D6 in FIG. 18) of the internal electrodes 15' and 16' disposed in the electrodes are optimized to equalize the main lens aperture effectively to about twice that of the aforementioned R dimension in all directions for the center and side electron beams, thereby to balance the focusing characteristics.
If the balance of these focusing characteristics collapses in one direction, the electron beam fails to be focused in that direction. Therefore, the focusing characteristics can be improved by enlarging the R dimension and accordingly the main lens aperture, thereby to reduce the spherical aberration. In the prior art described above, however, the R dimension is restricted within the S dimension.
Incidentally, Japanese Patent Publication No. 5591/1974 discloses an electron gun for a color cathode ray tube, which is given a large aperture lens by causing the three electron beams to intersect in the single cylindrical type main lens portion.
FIG. 21 is a schematic section for explaining a schematic structure of an electron gun for a color cathode ray tube of the prior art, which is given a large aperture lens by causing the three electron beams to intersect in the single cylindrical type main lens portion. The same reference numerals as those of FIG. 18 correspond to identical portions in FIG. 21. Numeral 20 designates deflection means, and letters BR, BG and BB designate the electron beams which land on the red, green and blue fluorescent elements, respectively.
In an electron gun of this type, as apparent from FIG. 21, the S dimension of the main lens portion is minimized because the three electron beams BR, EG and BB are made to intersect in the main lens. Downstream of the main lens portion, the two side electron beams BR and BB have to be diverged again to such an S dimension in the position of the deflection means 20 for converging the two side electron beams in such a way as to cause no deterioration of the aforementioned purity.
For this purpose, the electrode (i.e., the fifth grid electrode 15) to be supplied with a high voltage, which has a space for gradually enlarging the gap between the two side electron beams BR and BB and which constitutes the main lens, has to be axially elongated to a predetermined value or more. Thus, there arises a defect that the electron gun has its overall length increased.